The Problem: According to the commandments of the major religions, God expects humans to have only a single lifelong romantic partner and to remain sexually faithful to them: "Thou shalt not commit adultery" (Exodus 20:14). Yet, as any given week of tabloid headlines will tell you, humans aren't naturally wired for monogamy. Even after we're married or in a monogamous relationship, the sex drive continues functioning, often producing strong feelings of attraction and lust for people other than one's chosen partner. Even celebrities and politicians in high-profile relationships, people who have by far the most to lose from being caught cheating, seem unable to resist the urgings of adulterous desire.
For his answer, which adds to what
I previously suggested, read it
here.